Index of content:
Volume 103, Issue 1, January 1998
- SELECTED RESEARCH ARTICLES 
103(1998); http://dx.doi.org/10.1121/1.423233View Description Hide Description
An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental results were consistent with theory.
103(1998); http://dx.doi.org/10.1121/1.423234View Description Hide Description
Snapping shrimp are among the major sources of biological noise in shallow bays, harbors, and inlets, in temperate and tropical waters. Snapping shrimp sounds can severely limit the use of underwater acoustics by humans and may also interfere with the transmission and reception of sounds by other animals such as dolphins, whales, and pinnipeds. The shrimp produce sounds by rapidly closing one of their frontal chela (claws), snapping the ends together to generate a loud click. The acoustics of the species Synalpheus paraneomeris was studied by measuring the sound produced by individual shrimp housed in a small cage located 1 m from an H-52 broadband hydrophone. Ten clicks from 40 specimens were digitized at a 1-MHz sample rate and the data stored on computer disk. A low-frequency precursor signature was observed; this previously unreported signature may be associated with a “plunger” structure which directs a jet of water forward of the claw during a snap. The peak-to-peak sound pressure level and energy flux density at 1 m (source level and source energy flux density) varied linearly with claw size and body length. Peak-to-peak source levels varied from 183 to 189 dB re: 1 μPa. The acoustic power produced by a typical snap was calculated to be about 3 W. A typical spectrum of a click had a low-frequency peak between 2 and 5 kHz and energy extending out to 200 kHz. The spectrum of a click is very broad with only a 20-dB difference between the peak and minimum amplitudes across 200 kHz. A physical model of the snapping mechanism is used to estimate the velocity, acceleration, and force produced by a shrimp closing its claws.